Circle of Confusion

Bruce Osgood · Nov 22, 2004

I'm learning that when figuring depth of focus, CoC needs to be determined/known for the given lens in use and .... well, a lot of other stuff.

Can you tell me if CoC is something calculated by testing like an EI? Or is it something static, permanent, uncontrollable and fixed within a lenses dimensions? ie: A 150 lens has a CoC while a 250 has another and these are inherent to this lens? Or does CoC need to calculated? If so how?

TIA, running in circles of confusion

Loose Gravel · Nov 22, 2004

CoC is something chosen (within limits) to meet your requirements. Try this:

Dead Link Removed

David A. Goldfarb · Nov 22, 2004

Acceptible CoC is relative to format, not the lens, and it's calculated in a few different ways. I'm sure there's someone here who can explain the math better than I can, but it's based on how much detail the eye can resolve and the typical enlargement factor and print-viewing distance with any particular format.

The simplest method of computing acceptible CoC is the Zeiss method, which just uses a constant (that conceals other complicated calculations about the resolving power of the human eye, average print size, and average viewing distance)--

CoC for any given format=diagonal of the format/1730

Of course you may not use a "typical" enlargement factor, so there's a subjective element to what is "acceptible." You may decide that the acceptible CoC is smaller than what is usually recommended for the format, if you make big prints, or alternately you might decide to set the lens at f:16 but follow the DOF guidelines for f:8--this would amount to the same thing.

sanking · Nov 22, 2004

Bruce (Camclicker) said:
I'm learning that when figuring depth of focus, CoC needs to be determined/known for the given lens in use and .... well, a lot of other stuff.

Can you tell me if CoC is something calculated by testing like an EI? Or is it something static, permanent, uncontrollable and fixed within a lenses dimensions? ie: A 150 lens has a CoC while a 250 has another and these are inherent to this lens? Or does CoC need to calculated? If so how?

TIA, running in circles of confusion

CoF is not fixed. It is determined based on various assumptions, of which the two most important are 1) observation distance, and 2) the theoretical limits of human resolution.

It is generally accepted that the optimum viewing distance for the human eye is approximately 10", or 25cm. At this distance the absoulte maximum limit of human resolution for persons with the very best vision is 15-20 lppm, but 10-15 lppm is more common. However, most CoF calculations are made based on an assumption of about 5 lppm at normal observation distance, which varies according to size of print.

Sandy King

Bruce Osgood · Nov 22, 2004

Loose, David and Sandy;

thank you very much. I better understand my questions now.

Alex Hawley · Nov 22, 2004

Bruce (Camclicker) said:
Or does CoC need to calculated? If so how?
TIA, running in circles of confusion

Honestly Bruce, I don't know why CoC needs to be calculated unless one is designing lenses. As for taking pictures, the image is in focus on the ground glass or out of focus. I have never seen a need to worry about it. Maybe I'm running in my own circle of cunfusion.

Max · Nov 23, 2004

CoC is useful for making a DIY DOF scale.

(just wanted to see how many acronyms I could use )

Graeme Hird · Nov 23, 2004

I thought a Circle of Confusion was a group of photographers standing around discussing depth of field.

I could be wrong though .....

Ole · Nov 23, 2004

No Graeme,
A Circle of Confusion is a group of photographers discussing the effect of Merklinger's Hinge Rule on the DOF and the feasibility of the hyperfocal plane.

Graeme Hird · Nov 23, 2004

My bad ...

BBarlow690 · Nov 23, 2004

Schneider's web site already has pretty good DOF tables for many lens lengths. If your favorite aperture isn't there, the extrapolation is linear and easily digitized on a hand calculator (using your digits...).

scootermm · Nov 23, 2004

I once read a definition for Circle of Confusion...... (or maybe I read it twice - at least)

and I dont think there is any better way to label said item.

rbarker · Nov 23, 2004

If you really want to confuse that circle of photographers discussing DOF (I'm with you on this, Graeme ), ask them to consider the effect of the bokeh of the lens on the CoC it produces. (insert evil chuckle here)

Ole · Nov 23, 2004

rbarker said:
If you really want to confuse that circle of photographers discussing DOF (I'm with you on this, Graeme ), ask them to consider the effect of the bokeh of the lens on the CoC it produces. (insert evil chuckle here)

You are evil. I'll give one hint - a Heliar gives greater perceived DOF than a Xenar of the same focal length at the same aperture.
And DOF is only perceived anyway, so it must be true

Graeme Hird · Nov 23, 2004

rbarker said:
If you really want to confuse that circle of photographers discussing DOF (I'm with you on this, Graeme ), ask them to consider the effect of the bokeh of the lens on the CoC it produces. (insert evil chuckle here)

Ohh, that's mean Ralph. There are going to be a whole bunch of anally retentive photographers bursting with ..... "data" while they debate the merits of near and far bokeh on this one. You're a cruel one, Mr Barker.

rbarker · Nov 23, 2004

Cruel, evil and mean. That's me.

TPPhotog · Nov 23, 2004

I always thought the Circle of Confusion was caused when you tell someone to go ....... in the corner when they were in a barrel

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